# -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
#
# Copyright (C) 2017 Canonical Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 3 as
# published by the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
from snapcraft.file_utils import calculate_hash
from . import errors


def split_checksum(source_checksum):
    """Splits source_checksum into algorithm and hash.
    :raises ValueError: if source_checksum is not of the form algorightm/hash.
    :returns: a tuple consisting of the algorithm and the hash.
    """
    try:
        algorithm, digest = source_checksum.split('/', 1)

    except ValueError:
        raise ValueError('invalid checksum format: {!r}'
                         .format(source_checksum))
    return (algorithm, digest)


def verify_checksum(source_checksum, checkfile):
    """Verifies that checkfile corresponds to the given source_checksum.
    :param str source_checksum: algorithm/hash expected for checkfile.
    :param str checkfile: the file to calculate the sum for with the
                          algorithm defined in source_checksum.
    :raises ValueError: if source_checksum is not of the form algorightm/hash.
    :raises DigestDoesNotMatchError: if checkfile does not match the expected
                                     hash calculated with the algorithm defined
                                     in source_checksum.
    :returns: a tuple consisting of the algorithm and the hash.
    """
    algorithm, digest = split_checksum(source_checksum)

    calculated_digest = calculate_hash(checkfile, algorithm=algorithm)
    if digest != calculated_digest:
        raise errors.DigestDoesNotMatchError(digest, calculated_digest)

    return (algorithm, digest)
